Electrochemical fuel cells convert fuel and oxidant to electricity and reaction product. Fluid reactants are supplied to a pair of electrodes which are in contact with and separated by an electrolyte. The electrolyte may be a solid or a liquid (supported liquid matrix). Solid polymer electrochemical fuel cells generally employ a membrane electrode assembly comprising a solid ionomer or ion-exchange membrane disposed between two planar electrodes. The electrodes typically comprise an electrode substrate and an electrocatalyst layer disposed upon one major surface of the electrode substrate. The electrode substrate typically comprises a sheet of porous, electrically conductive material, such as carbon fiber paper or carbon cloth. The layer of electrocatalyst is typically in the form of finely comminuted metal, typically platinum, and is disposed on the surface of the electrode substrate at the interface with the membrane electrolyte in order to induce the desired electrochemical reaction. In a single cell, the electrodes are electrically coupled to provide a path for conducting electrons between the electrodes through an external load.
At the anode, the fuel moves through the porous anode substrate and is oxidized at the anode electrocatalyst layer. At the cathode, the oxidant moves through the porous cathode substrate and is reduced at the cathode electrocatalyst layer.
Electrochemical fuel cells most commonly employ gaseous fuels and oxidants, for example, those operating on molecular hydrogen as the fuel and oxygen in air or a carrier gas (or substantially pure oxygen) as the oxidant. The anode and cathode reactions in hydrogen/oxygen fuel cells are shown in the following equations: EQU Anode reaction:H.sub.2 .fwdarw.2H.sup.+ +2e.sup.- EQU Cathode reaction:1/2O.sub.2 +2H.sup.+ +2e.sup.- .fwdarw.H.sub.2 O
The catalyzed reaction at the anode produces hydrogen cations (protons) from the fuel supply. The ion-exchange membrane facilitates the migration of protons from the anode to the cathode. In addition to conducting protons, the membrane isolates the hydrogen-containing gaseous fuel stream from the oxygen-containing gaseous oxidant stream. At the cathode electrocatalyst layer, oxygen reacts with the protons that have crossed the membrane to form water as the reaction product.
In liquid feed electrochemical fuel cells, one or more of the reactants is introduced to the electrocatalyst in the liquid form. Examples of electrochemical fuel cells which can be operated with a liquid fuel feed are those employing a lower alcohol, most commonly methanol, as the fuel supplied to the anode (so-called "direct methanol" fuel cells) and oxygen to the cathode. In fuel cells of this type the reaction at the anode produces protons, as in the hydrogen/oxygen fuel cell described above, however the protons (along with carbon dioxide) arise from the oxidation of methanol. An electrocatalyst promotes the methanol oxidation at the anode. The methanol may alternatively be supplied to the anode as vapor, but it is generally advantageous to supply the methanol to the anode as a liquid, preferably as an aqueous solution. In some situations, an acidic aqueous methanol solution is the preferred feed to the anode. The anode and cathode reactions in a direct methanol fuel cell are shown in the following equations: EQU Anode reaction:CH.sub.3 OH+H.sub.2 O .fwdarw.6H.sup.+ +CO.sub.2 +6e.sup.- EQU Cathode reaction:3/2O.sub.2 +6H.sup.+ +6e.sup.- .fwdarw.3H.sub.2 O EQU Overall reaction:CH.sub.3 OH+3/2O.sub.2 .fwdarw.CO.sub.2 +2H.sub.2 O
The protons formed at the anode electrocatalyst migrate through the ion-exchange membrane from the anode to the cathode, and at the cathode electrocatalyst layer, the oxidant reacts with the protons to form water.
Other non-alcohol fuels may be used in liquid feed fuel cells, for example formic acid. The oxidant may also be supplied as a liquid, for example, as an organic fluid with a high oxygen concentration (see U.S. Pat. No. 5,185,218), or as a hydrogen peroxide solution.
In electrochemical fuel cells employing liquid or solid electrolytes and gaseous or liquid reactant streams, crossover of a reactant from one electrode to the other is generally undesirable. Reactant crossover may occur if the electrolyte is permeable to the reactant, that is, some of a reactant introduced at a first electrode of the fuel cell may pass through the electrolyte to the second electrode, instead of reacting at the first electrode. Reactant crossover typically causes a decrease in both reactant utilization efficiency and fuel cell performance. Fuel cell performance is defined as the voltage output from the cell at a given current density or vice versa; the higher the voltage at a given current density or the higher the current density at a given voltage, the better the performance.
In solid polymer electrochemical fuel cells the ion-exchange membrane may be permeable to one or more of the reactants. For example, ione-xchange membranes typically employed in solid polymer electrochemical fuel cells are permeable to methanol, thus methanol which contacts the membrane prior to participating in the oxidation reaction can cross over to the cathode side. Diffusion of methanol fuel from the anode to the cathode leads to a reduction in fuel utilization efficiency and to performance losses (see, for example, S. Surampudi et al., Journal of Power Sources, vol. 47, 377-385 (1994) and C. Pu et al., Journal of the Electrochemical Society, vol. 142, L119-120 (1995)).
Fuel utilization efficiency losses arise from methanol diffusion away from the anode because some of the methanol which would otherwise participate in the oxidation reaction at the anode and supply electrons to do work through the external circuit is lost. Methanol arriving at the cathode may be lost through vaporization into the oxidant stream, or may be oxidized at the cathode electrocatalyst, consuming oxidant, as follows: EQU CH.sub.3 OH+3/2O2.fwdarw.CO.sub.2 +2H.sub.2 O
Methanol diffusion to the cathode may lead to a decrease in fuel cell performance. The oxidation of methanol at the cathode reduces the concentration of oxygen at the electrocatalyst and may affect access of the oxidant to the electrocatalyst (mass transport issues). Further, depending upon the nature of the cathode electrocatalyst and the oxidant supply, the electrocatalyst may be poisoned by methanol oxidation products, or sintered by the methanol oxidation reaction.
The electrode structures presently used in direct methanol solid polymer fuel cells were originally developed for hydrogen/oxygen fuel cells. The anode electrocatalyst which promotes the oxidation of methanol to produce protons is typically provided as a thin layer adjacent to the ion-exchange membrane (see U.S. Pat. Nos. 5,132,193 and 5,409,785 and European Patent Publication No. 0090358). The anode electrocatalyst layer is typically applied as a coating to one major surface of a sheet of porous, electrically conductive sheet material or to one surface of the ion-exchange membrane. This provides a limited reaction zone in which the methanol can be oxidized before contacting the membrane electrolyte. Thus, with this type of electrode, the methanol concentration at the anode-electrolyte interface will typically be high.
Reactant crossover may be substantially eliminated if a reactant introduced to a first major surface of a fuel cell electrode is substantially completely reacted on contacting the second major surface of the electrode. In this case essentially no unreacted reactant would be available to pass from the second surface through the electrolyte to the other electrode. As described herein, this may be accomplished by ensuring that the reactant contacts sufficient catalyst so that it is substantially completely reacted before it contacts the second surface of the electrode.
It is therefore an object of the invention to provide an electrochemical fuel cell in which crossover of a reactant from one electrode to the other is reduced.
It is a further object of the invention to provide a solid polymer electrochemical fuel cell in which a reactant is substantially completely reacted before it contacts the membrane electrolyte.
It is a still further object of the invention to provide a direct methanol solid polymer fuel cell in which methanol crossover from the anode to the cathode is reduced.
Another object of the invention is to provide a method for reducing reactant crossover in an electrochemical fuel cell.